1. Field of the Invention
The present invention relates to color image forming apparatus such as color printers, color copying machines, and so on and, more particularly, to a method of making correction for color sensor output values in color image forming apparatus, and color image forming apparatus, intended for improvement in color reproducibility.
2. Description of the Related Art
In recent years, there has been demand for achievement of higher image quality of output images in color image forming apparatus employing the electrophotographic system, the ink jet system, and the like, such as color printers, color copying machines, and so on.
Especially, the gradation of density and the stability thereof have a large effect on a person's judgment of the quality of an image.
However, a color image forming apparatus varies the density of the images it produces with changes in the environment of the apparatus and with variation of parts of the apparatus due to long-term use.
Particularly, in the case of a color image forming apparatus that uses the electrophotographic system, even a small environmental variation produces a significant variation in the image density, so as possibly to disturb the color balance, and there is thus a need for having means for maintaining density-gradation characteristics constant.
The apparatus is thus provided with process conditions such as several types of exposure amounts, development biases, etc., according to absolute humidities, and a gradation correcting means such as a look-up table (LUT) or the like, for each of the toners of the respective colors used as colorants in the apparatus, and based on the absolute humidity as measured by a temperature-humidity sensor, the apparatus selects the process conditions and the optimal value of gradation correction at that time.
In order to obtain constant density-gradation characteristics even with variation in each part of the apparatus, the apparatus is configured to form a density-detection toner patch of the toners of the respective colors on an intermediate transferring member, a drum, or the like, detect the density of the unfixed toner patch with an unfixed-toner-density detecting sensor (hereinafter referred to as a “density sensor”), and perform feedback density control to feed the result of the detection back to the process conditions of exposure amounts, development biases, etc., thereby obtaining stable images.
However, the density control with the density sensor is based on the detection of the patch formed on the intermediate transferring member, the drum, or the like, and no attempt is made tp control changes in the color balance of an image that may occur in the transferring and fixing of the colorants onto a transferring material.
In particular, density control using a density sensor is not readily adaptable to controlling changes in color balance due to transfer efficiency in transferring the toner image onto the transferring material and due to heating and pressing during fixing.
It is thus contemplated to employ a correction method of forming a gray patch of black (K) and a process gray patch of cyan (C), magenta (M), and yellow (Y) mixed, not on the intermediate transferring member, the drum, or the like, but on the transferring material, comparing colors of the two patches after they have been fixed, relative to each other, thereby detecting any change that may have occurred in the color balance after fixing, and making correction based thereon.
For example, there are a color image forming apparatus equipped with a sensor for detecting if the process gray patch has turned into aromatic color (hereinafter referred to as a “color sensor”), and configured to make correction based on a CMY mixture ratio upon the process gray patch turning achromatic.
The color image forming apparatus of this type is arranged to feed the result of the detection back to exposure amounts and process conditions in an image forming portion, and to a color-matching table for converting RGB signals of an image processing portion into a color reproducible region of the color image forming apparatus, a color separation table for converting RGB signals into CMYK signals, a calibration table for correction for density-gradation characteristics, and so on, whereby the apparatus is able to perform control on the density or chromaticity of the final output image formed on the transferring material.
Although it is possible to perform similar control by detecting the output image of the color image forming apparatus with an external image reading device or with a color meter or a density meter, the above-stated method is superior in that the control is complete in the printer.
This correction method will be described below specifically.
For the purpose of making correction for deviations due to secular change of the sensor or the like, the conventional correction method was arranged so that when the color sensor had an absolute white (color) reference board, the correction for maintaining the balance among RGB output values was carried out utilizing RGB output values obtained by detecting the absolute white reference board with the color sensor; in a case where the color sensor did not have the absolute white reference board, the correction for maintaining the balance among RGB output values was carried out utilizing RGB output values upon detection of a transferring material assumed to be absolute white, with the color sensor.
First, the correction method using an absolute white reference board will be described with reference to FIG. 14.
Ssi represents the respective measured values of RGB outputs of the absolute white reference board; Sf0i represents the respective theoretical values of RGB outputs; Si represents the respective measured values of RGB outputs of the patch; and Si′w represents the respective RGB output values resulting from the correction using the absolute white reference board.
The first step is to detect the RGB output measured value Ssi (here and in the following description, i=r, g, or b) of the absolute white reference board (S1401).
The next step is to obtain a correction coefficient Sf0i/Ssi from the RGB output measured value Ssi of the absolute white reference board as detected and the RGB output theoretical value Sf0i preliminarily stored (S1402).
The subsequent step is to obtain the RGB output measured value Si of the patch (S1403).
Then, according to Eqn. (1) below, the RGB output measured value Si of the patch thus obtained is uniformly multiplied by the correction coefficient Sf0i/Ssi and is thus converted into the RGB output value Si′w based on the correction using the absolute white reference board (S1404):Si′w=Si×(Sf0i/Ssi) (i=r, g, or b)  (1)
The next step is to output the RGB output value Si′w resulting from the correction using the absolute white reference board (S1405).
FIG. 16 shows the relationship among toner bearing amount, RGB output theoretical value, RGB output measured value Si of the patch, and RGB output value Si′w resulting from the correction using an absolute white reference board.
Curve (1) indicates the RGB output theoretical value, curve (2) indicates the RGB output measured value Si of the patch, and curve (3) indicates the RGB output value Si′w resulting from the correction using an absolute white reference board.
Although there are in fact three output values, respectively of R, G, and B, these separate values are abbreviated by i (i=r, g, or b), as mentioned, because the correction method is common thereto.
The objective herein is to correct the RGB output measured value Si of the patch toward the RGB output theoretical value, i.e., to bring the correction result of curve (2) to a location as close to curve (1) as possible.
It can be said that curve (3), being the correction result using an absolute white reference board, is located closer to curve (1), although there is still some difference.
Next, the correction method using the transferring material will be described referring to FIG. 15.
S0i indicates the respective measured values of RGB outputs of the transferring material; and Si′p the respective RGB output values resulting from the correction using the transferring material.
The first step is to detect the RGB output value S0i of the transferring material (S1501).
The next step is to obtain a correction coefficient Sf0i/S0i from the RGB output value S0i of the transferring material as detected and the RGB output theoretical value Sf0i preliminarily stored (S1502).
The subsequent step is to obtain the RGB output measured value (Si) of the patch (S1503).
Then, according to Eqn. (2) below, the RGB output measured value Si of the patch obtained is uniformly multiplied by the correction coefficient Sf0i/S0i and thus is converted into the RGB output value Si′p based on the correction using the transferring material (S1504):Si′p=Si×(Sf0i/S0i) (i=r, g, or b)  (2)
The next step is to output the RGB output value Si′p resulting from the correction using the transferring material (S1505).
FIG. 16 shows the relationship among toner bearing amount, RGB output theoretical value, RGB output measured value Si of the patch, and RGB output value Si′p resulting from the correction using the transferring material.
Curve (1) represents the RGB output theoretical value, curve (2) represents the RGB output measured value Si of the patch, and curve (4) represents the RGB output value Si′p resulting from the correction using the transferring material.
It can be mentioned that curve (4), being the correction result using the transferring material is located closer to curve (1), although there again still remains some difference.
The color sensor used in these correction methods is arranged to obtain three or more different outputs, such as the RGB outputs or the like, for example, in a configuration where three or more light sources with different emission spectra of red (R), green (G), blue (B), etc., are provided as light emitting devices, or in a configuration wherein a light source emitting light of white color (W) is used as a light emitting device and three or more filters with different spectral transmittances of red (R), green (G), blue (B), etc., are provided on a light receiving device.
In the case of printers of the ink jet type, the color balance also differs depending upon secular change and environmental differences that cause variations in the ink discharge amounts, and upon individual differences among ink cartridges, so that the density-gradation characteristics are not maintained constant; therefore, it is contemplated that control of density or chromaticity may be performed by setting the color sensor near the output portion of the printer and detecting the density or chromaticity of the patch on the transferring material.
As described below, correction was made heretofore for the deviation of the color sensor output values due to the color sensor as color detecting means, using an absolute white reference board or the transferring material.
However, the chromaticity of the patch formed in execution of the density or chromaticity control with the color sensor is affected by the chromaticity of the transferring material.
Therefore, even if the same patch is formed on different transferring materials, the chromaticities of the patches will be affected by those transferring materials, and thus the detection results upon detection of the patch with the color sensor will not be the same in each case.
Especially, a low-density patch is more likely to be affected by the color of the transferring material, because the transferring material is exposed more.
This was responsible for a drop in accuracy of the correction for RGB output values and further induced degradation of color reproducibility of the color image forming apparatus.
Describing it referring to FIG. 16, curve (3) of the RGB output value resulting from the correction using the absolute white reference board increases the difference from curve (1) of the RGB output theoretical value with decrease in the toner bearing amount, because the transferring material is not completely white.
Curve (4) of the RGB output value resulting from the correction using the transferring material agrees with curve (1) of the RGB output theoretical value at the toner bearing amount of 0, but increases the difference from curve (1) of the RGB output theoretical value in the toner bearing range except at the value 0.
Namely, even with correction by either of these methods, a difference occurred between curve (3) or (4) of the correction RGB output value and curve (1) of the RGB output theoretical value because of the influence of the color of the transferring material.